Power supply systems, in particular systems which generate power at frequencies of >1 MHz and in particular less than 200 MHz and primarily in particular less than 90 MHz, are used for example for laser excitation or in plasma coating installations. Often, in power supply systems of this type, a plurality of amplifiers are used so as to generate a total power of the power supply system therefrom.
In a power supply system of this type, abrupt changes in the required power may occur, for example, if an arc occurs in the plasma chamber and the supplied power accordingly has to be reduced abruptly. On the other hand, the power required for igniting a plasma may be different from one for operating a plasma process. When the plasma state changes, so does the impedance of the load. This likewise results in an abrupt change in the load. It is often not possible to adjust the impedance sufficiently rapidly, and so the power is reflected by the load. The reflected power should, if possible, be kept away from the amplifiers or amplifier paths so as to prevent destruction or damage to the power supply system. It is known to use circulators to absorb the reflected power. However, in the stated frequency range, these circulators are very bulky and are no longer practical to use.
If abrupt load changes occur along with an abrupt change in the target power, the output power has to be controlled. This may be done for example by varying the input power of an amplifier. In the process, however, the efficiency of the amplifier also changes. At small output powers, the amplifier will operate in the back-off range. This is the range in which the amplifier only provides part of the maximum possible output power at the output. In this back-off range, the efficiency of the amplifier is reduced. Depending on the load to be operated, the transistor used in the amplifier therefore has to dissipate more lost power. It therefore becomes much warmer. In the event of mismatching, this behavior changes. Depending on the load angle or reflection angle, the transistor becomes warmer either in the back-off range or in the saturation range. It is therefore only possible to control the output power by way of the input power to a certain extent in the event of mismatching. To prevent the transistor from overheating, it has often been recommended to limit the maximum output power. However, lowering the maximum output power does not solve this problem, since, in the event of a major mismatching and an unfavorable load angle, critical temperatures occur even at low powers, not just at relatively high powers.
Thus far, a viable option has been to regulate the supply voltage of the transistor and in doing so to reduce it in such a way that it is possible to pass through the entire characteristic range at a reduced saturated power of the amplifier. If the voltage supply is too slow, the voltage supply has to remain set to relatively high voltages even at low output powers. In this case, sudden load changes may lead to excessive heating of the transistor in the amplifier.